Introduction

What is an application server?
          As you well know, compiled Java code cannot run on its own--it must be run inside of a JVM. J2EE classes require an additional step: they must be run inside of a J2EE container (which in turn runs inside of a JVM). The container provides an infrastructure and a framework in which your code runs. It handles the mundane low-level "plumbing" aspects of the application, freeing you to focus on the higher-level business development. An application server is nothing more than a loosely coupled collection of containers (or "services") that correspond to the various parts of the J2EE API.

The beauty of the J2EE is that it is a specification, not an implementation. In other words, Sun provides the technical guidelines for how each service should behave, but leaves it open for anyone to write a server that can host a J2EE application. This gives you, the developer, a wide variety of choices when it comes to selecting an application server. While the details of how to deploy a J2EE application may vary from server to server, you can rest assured that your compiled code will behave the same way regardless of the application server it is running in.

JBoss is an open source J2EE application server. It is J2EE 1.4 spec-compliant, which means that it offers the same level of functionality as its more expensive commercial counterparts. What this means to you is that the lessons learned here will be largely useful, regardless of the application server you are currently using. The most important thing to keep in mind is that cost does not equal value or reliability. The open source Apache web server runs more than two-thirds of all web sites today. JBoss isn't quite at that level of pervasiveness, but it shouldn't be ignored when considering enterprise-quality application servers just because it is free.